Within the field of surgical repair of soft tissue defects, use is often made of a mesh implant made of a non-resorbable material that is inserted to cover the area of the tissue defect. The mesh implant is used in order to support the regenerating tissue, and in, e.g. hernia defects, it works by mechanical closure of the defect and by inducing a strong scar fibrous tissue around the mesh implant. Such a mesh implant is most often made of various plastics, which are known to stay biostable and safe for a number of years after implantation. However, introducing a foreign material into the human or animal body is most often accompanied with side effects like migration, chronic inflammation, risk of infection etc. The introduction of a relatively large plastic body is also likely to induce a foreign body-reaction caused by the body's immune defence system. As a result, the mesh implant may crumple up and loose its tissue supporting function.
The above mentioned mesh implants are in particular used in the repair of defects in the abdominal wall, which may be a result from trauma, tumour resection, prolapse or hernia.
A hernia is an abnormal protrusion of a peritoneal-lined sac through the musculoaponeurotic covering of the abdomen, the most common site for a hernia being the groin. Types of hernia are, among others, inguinal hernia or a femoral hernia, hiatal hernia, umbilicial hernia and incisional hernia, the latter being a hernia that pushes through a past surgical incision or operation.
One suggested theory in the field is that some patients, due to collagen metabolic disorders, have a genetic predisposition for developing recurrent hernias. An altered ratio of collagen types I and III in these patients, with an increase in collagen type III, is believed to reduce the mechanical strength of connective tissues. The decreased tensile strength of collagen type III plays a key role in the development of incisional hernias, see KLINGE, U, et al. Abnormal collagen I to III distribution in the skin of patients with incisional hernia. Eur Surg Res. 2000, vol. 32, no. 1, p. 43-48.
It is in particular in the cases of large or recurrent hernias that the surgical repair or herniorrhaphy makes use of an inert, non-resorbable mesh implant, as described above. The mesh implant is inserted to cover the area of the abdominal wall defect without sewing together the surrounding muscles. This can be done under local or general anesthesia using a laparoscope or an open incision technique.
Among the laparoscopic techniques used, are the trans-abdominal pre-peritoneal (TAPP) technique and the totally extra-peritoneal (TEP) technique. With the TAPP technique, the pre-peritoneal space is accessed from the abdominal cavity, whereupon the mesh implant is placed between the peritoneum and the transversalis fascia. With the TEP technique, the mesh implant is again placed in the retroperitoneal space, but the space is accessed without violating the abdominal cavity. An open and minimal invasive technique is the Lichtenstein hernia repair technique, in which the upper edge of the mesh implant is attached to the outer side of the internal oblique and the lower edge of the mesh implant is attached to the aponeurotic tissue covering the pubis.
Another open minimal invasive technique is the mesh-plug technique comprising attaching a mesh implant, as described above in reference to the Lichtenstein technique, but also inserting a plug pushing the peritoneum in a direction towards the abdominal cavity.
The mesh implant, inserted with any of the above described techniques, is used in order to support the regenerating tissue with minimal tension. It works by mechanical closure of the defect in the abdominal wall and by inducing a strong scar tissue around the mesh implant fibres. The commercially available hernia mesh implants are often made of various, inert, non-resorbable polymeric materials, typically polypropylene, and suffers from the same disadvantages, as described above in connection with mesh implants used for reconstruction of soft tissue defects in general. However, implantation of large pieces of mesh implants in the abdominal wall cavity, also leads to considerable restriction thereof. In a study performed by Junge et al, JUNGE, K, et al. Elasticity of the anterior abdominal wall and impact for reparation of incisional hernias using mesh implants. Hernia. 2001, no. 5, p. 113-118., the elasticity of the abdominal wall was measured and compared to that of commercially available non-resorbable hernia mesh implants. It was assumed that the flexibility of the abdominal wall is restricted by extensive implantation of large mesh implants, the more so if the mesh implants are integrated into scar tissue. In addition, the non-physiological stretching capability of the mesh implants contrast with the highly elastic abdominal wall and can give rise to shearing forces, favouring increased local remodelling and thus recurrence at the margin. It was concluded that mesh implants used for repairing inscisional hernia should have an elasticity of at least 25% in vertical stretching and 15% in the horizontal stretching when subjected to a tensile strength of 16 N/cm, in order to achieve almost physiological properties.
The progress within hernia repair mesh implant development, as well as in the development of mesh implants for the use of reconstruction of soft tissue defects in general, has been towards mesh implants with less mass in order to minimize foreign body reactions, and larger pore sizes, which on one hand reduce the mass of the mesh implant and on the other facilitate ingrowth of tissue.
U.S. Pat. No. 6,319,264 B (TÖRMÄLÄ) 20.11.2001 describes a porous, flexible and fibrous hernia mesh, which is intended to be implanted close to hernia defects. The mesh comprises two functional layers, wherein the first layer is a rapidly degradable polymer layer facing the fascia, and wherein the second layer is a more slowly degradable polymer layer. The first polymer layer has a fast resorption profile, approximately 14 days, said first layer promotes scar tissue formation due to inflammatory reactions induced by the polymer degradation and due to the porous structure of the first layer. The second polymer layer has a longer resorption time, approximately 6 months, and thus supports the area until the scar tissue is strong enough to resist pressure and prevent recurrent hernia formation. An optional third dense, thin, bioabsorbable layer is described, which prevents agents that could cause tissue to tissue adhesion from moving from the hernia area through the mesh and onto the surrounding tissue during the first weeks after the operation. The mesh described in U.S. Pat. No. 6,319,264 acts as a temporary support until connective scar tissue has strengthened enough and can replace the mesh, when the second layer finally degrades.
However, U.S. Pat. No. 6,319,264 is silent as to the load situation found over the tissue defect area and to the modulus of elasticity of the hernia mesh. In the above described mesh, only the second layer is designed to support the tissue during the regenerative phase. The mesh material is by the body regarded as an inert material, in that no major changes in mechanical properties are observed until degradation has reached to such a point where the material starts to crack with a more or less catastrophic change in mechanical properties taking place.
The complex but well orchestrated sequence in wound healing starts with hemostasis and the wound is usually fully closed within 10 to 14 days. However, depending on the individual and the size of the wound, the healing sequence may be faster or slower. This is especially true if the wound is infected. Collagen remodelling and deposition is however a continuous process slowly building up the strength in the wounded tissue. Fibroblasts play a key role in the early phase of the wound-healing and are present already from day 2 or 3. The key role of the fibroblasts is to deposit new collagen into the wounded area rebuilding the extra-cellular matrix and thus repairing the wounded tissue. This first deposited collagen is most often laid down in a random non-oriented fashion and is often referred to as scar tissue. However, by stimulating the wound already during the early or acute phase of wound healing we have reason to believe that a denser and stronger collagen layer is deposited. These findings implicate that the surgical mesh used for soft tissue reconstruction should possess properties that would allow the mesh to become more compliant with the surrounding tissue during the early phase of wound healing and a continuous increase in compliance could be visualized after the remodelling phase starts. After the acute wound-healing phase is over, the wound is often remodelled, i.e. the first deposited collagen is replaced by a more structurally rigid collagen. During this remodelling period, the newly formed tissue will undergo several phases, during which the tissue gradually becomes more specific to support the various stress situations found in the area. Following the teachings of Junge et al, a mesh implant used for reconstruction of soft tissue defects, should have an elasticity that is compatible with the elasticity of the surrounding tissue, so that the flexibility of said tissue is not substantially restricted.
It is therefore believed that device compliance will play a key role in the remodelling sequence of the first deposited collagen. But to our surprise we have also found that meshes used for soft tissue repair and which alter their mechanical properties in the very early phase of the wound healing sequence rather than in the remodelling phase of the wound, may stimulate gross infiltration of collagen into the knitted mesh construct. This early infiltration of collagen may play a crucial role in the strength of the wound at a later stage. It appears that a polymeric mesh for reconstruction of soft tissue defects should be so designed that it will allow for early stimulation of the newly deposited tissue. This can be accomplished by allowing an early change in the modulus of the implant so that it gradually become more and more compliant to the surrounding tissue.
The inventors of the present invention therefore suggest that a device used to temporarily support the tissue defect in the area where the tissue is exposed to various stress situations should be so designed as to allow for an early change in the modulus of the implant, best expressed as an increased elongation, thus allowing an early stimulation of the deposited collagen in the wound area followed by a gradual change in compliance of the implant allowing the newly formed tissue to gradually take over the load during the remodelling phase and thus build up the strength and compliance needed to take over the full load once the support from the temporarily implanted device is lost.